This invention relates to a continuous process for producing blends of high viscosity silicone fluids and thermoplastic polymers. More particularly, this invention is related to a method of continuously blending one or more thermoplastic polymers and a high viscosity silicone fluid within an extruder.
Blending thermoplastic polymers with high viscosity silicone fluids often provides blends with desirable engineering properties and improved flame retardance, such as where the silicone fluid is a constituent of a flame retardant additive. Examples of such desirable blends are disclosed by MacLaury et al. in U.S. Pat. No. 4,273,691 and Frye in U.S. Pat. No. 4,387,176, assigned to the same assignee as the present invention.
Producing thermoplastic/silicone fluid blends has presented certain problems. The high viscosity silicone fluids typically comprise siloxane polymers having an average molecular weight of 50,000 or above and have a viscosity of 90,000 centipoise and above at 25.degree. C. These fluids are difficult to handle and feed into conventional blending equipment with solid thermoplastic polymers due to their high viscosity.
Thermoplastic/silicone fluid blends have been produced with batch processes by MacLaury et al. and Frye. In these batch processes two roll mills and high intensity mixers are utilized.
Extruders are typically utilized in conventional continuous blending processes. However, the production of thermoplastic/silicone fluid blends by such processes has been difficult, if not impossible to achieve. The highly viscous silicone fluids and solid thermoplastic polymers cannot be fed into a conventional extruder without premixing the constituents into a uniform solids-gum feedstock. The difference in the consistencies of the silicone fluids and thermoplastic polymers does not permit them to be fed simultaneously into an extruder without premixing.
This premixing is usually accomplished by a batch process, wherein the high viscosity silicone fluid and a portion of the solid thermoplastic polymers are blended in a two roll mill or dough mixer. The remainder of the solid thermoplastic polymer is then mixed in with a high intensity mixer. This mixture is often screened by hand to break up agglomerated solids to provide a uniform solids-gum feedstock. This solids-gum feedstock is suitable for providing a uniform feed into an extruder since the two constitutents cannot separate within the feed hopper.
It is desirable to obtain a process where blends of high viscosity silicone fluids and solid thermoplastic polymers can be produced continuously without the costly pretreatment of the blend components. The present invention is based on the discovery that high viscosity silicone fluids can be fed into an extruder downstream of the feed hopper without substantially affecting the degree of dispersion of the blend produced. Furthermore, it has been discovered that this process provides blends having improved engineering properties and flame retardance over blends produced by conventional continuous processes and batch processes.